The proposed project will consider (1) the role of platelet thrombosis in the genesis of venous thromboembolism in surgical patients, (2) the identification of aspects of platelet function relative to the above, and (3) the efficacy of pharmacologic agents affectng platelet function in the prevention of post-operative thromboembolism. Surgical patients at high risk of thromboembolism will be studied by sensitive tests for diagnosis of venous thrombosis and pulmonary embolism, and aspects of platelet function and other homostatic processes will be evaluated. Various drugs will be assessed for their effectiveness in prevention of venous thromboembolism, with particular emphasis on drugs affecting platelet function. These studies will furnish a clinical outlet for parallel laboratory investigation of platelet biochemistry and the pharmacology of antithrombotic drugs.